1. Field of the Invention
The present invention relates to an apparatus for rate control for a video cassette recorder (VCR) or the like for recording a digitized image data by compression coding technologies so that the code amount of the compression coded data becomes a predetermined code amount or less.
2. Description of the Related Art
A compression coding technology utilizing an orthogonal transform in combination with a variable-length coding is regarded as a high-efficiency compression coding technology for a natural image. The adoption of this method as an international standard of a color natural image coding method was decided (see the Journal of the Institute of Television Engineers of Japan, Vol 46, No. 8, pp. 1021-1024 (1992)).
In such a compression method, since the variable-length coding is performed, the code amount is varied in accordance with an image to be compressed. On the contrary, in a system such as a VCR, the recording operation is required to be performed at a constant code rate, and therefore, the code amount is required to be controlled for each image to be compressed. A rate control method for such a purpose is proposed, for example, in "Bit-rate Control for the JPEG Coding", 16-15, Annual Convention of The Institute of Television Engineers of Japan, 1992.
According to the method, first, a plurality of blocks in one field are linearly quantized using M (where, M is a natural number) different kinds of scale factors .alpha..sub.1 to .alpha..sub.M (where .alpha..sub.1 &lt;.alpha..sub.2 &lt;. . . . &lt;.alpha..sub.M) for the respective blocks. Then, the code amount of two-dimensional Huffman coded variable-length data in one block is calculated. At this time, the summation values N.sub.1 to N.sub.M of the M kinds of code amounts corresponding to the respective scale factors are calculated and a scale factor for setting the code amount of one field to be a predetermined value is predicted based on the relationship between the scale factor and the code amount.
On the other hand, an interleave coding method has been proposed separately from the above-described rate control method. According to the interleave coding method, in order to prevent a part of image data from being unavailable on a block basis even when an error is generated in the compressed data, the blocks are rearranged by interleaving the data among a plurality of DCT blocks, and the data is coded and transmitted in the rearranged interleaved blocks (see, for example, Japanese Patent Laid-Open Publication No. 7-298194).
Furthermore, in order to realize an adaptive compression coding in accordance with local features of an image, a method for improving the quality of the image by categorizing the respective DCT blocks based on the features of the image in the blocks and optimizing the quantization characteristics in accordance with the categories has been proposed (see, for example, "An Adaptive DCT Coding for HDTV Digital VCR", Convention of the Institute of Electronics, Information and Communication Engineers of Japan, D-220, autumn 1990).
However, in the case of applying the above-described rate control method to the interleave coding method in which the DCT blocks are interleaved, it is necessary to predict the code amounts of the interleaved blocks. Therefore, it is difficult to perform such a prediction coding by conventional technologies. On the other hand, it is possible to perform a pre-scanning after the interleave is performed. However, in such a case, a memory for delaying the interleaved data is also required to be provided in addition to a memory for performing the interleave.
Moreover, in the case of applying the rate control method to the adaptive processing method, the quantization characteristics become different in the respective DCT blocks. Therefore, it is impossible to perform the rate control by applying a uniform scale factor to the entire image unlike the above-described rate control method.